In most online dictionaries, if you look up 居る, it gives you two readings: いる, formerly ゐる, and おる, formerly をる.
However, I can't tell what the difference in connotation or usage would be. I couldn't find too many resources or articles talking about this.
Wiktionary states the following etymology:
Listed in some sources as derived from the 連用形 (ren'yōkei, “continuative or stem form”) wi of verb wiru (modern いる (iru)) + ある (aru). The conjugation paradigm for woru is indeed the same as aru for Classical and Old Japanese. However, the resulting compounded *wiaru form would ordinarily become yaru or eru via historical phonetic change, and the vowel shift to /o/ is unexplainable in this hypothesis.
Wiktionary also states basically the same definitions for both the verbs いる and おる.
[verb] (of animate objects) to exist, to be
[verb] (of animate objects) to have
[verb] (of animate objects) to exist, to be, to have
So what's the difference in usage between いる and おる? Is formality all there is to it? Or is there other differences? When should one use おる instead of いる?